Focussing on preplanetary grains growth , we discuss the properties of dust aggregation driven by magnetic dipole forces .
While there is no direct evidence for the existence of magnetic grains present in the solar nebula , there are reasons to assume they may have been present .
We derive analytical expressions for the cross-section of two interacting dipoles .
The effective cross section depends upon the strength of the magnetic dipoles and the initial velocities .
For typical conditions the magnetic cross section is between 2 and 3 orders of magnitude larger than the geometric cross section .
We study the growth dynamics of magnetic grains and find that the mass of the aggregates should increase with time like t ^ { 3.2 } whereas Brownian motion growth behaves like t ^ { 2 } .
A numerical tool is introduced which can be used to model dust aggregation in great detail , including the treatment of contact forces , aggregate restructuring processes and long-range forces .
This tool is used to simulate collisions between magnetic grains or clusters and to validate the analytical cross-sections .
The numerically derived cross section is in excellent agreement with the analytical expression .
The numerical tool is also used to demonstrate that structural changes in the aggregates during collisions can be significant .